Labour former Home Secretary Alan Johnson has insisted that Jeremy Corbyn is not up to the job of being leader of the opposition as the party was plunged into fresh in-fighting after a shadow Cabinet shake-up.

Johnson’s damning assessment came as the chairman of the parliamentary party, John Cryer, condemned Corbyn’s controversial shake-up of his top team.

When it was put to Johnson on BBC Radio Four’s Today programme that he believed Corbyn was not up to the leader’s job, he said: “Me and many of my colleagues, perhaps he’ll prove me wrong.”

As the backlash against Corb­yn’s reshuffle intensified, Cryer complained that he and sacked chief whip Rosie Winterton had been kept in the dark about the move despite holding talks with the leadership on having some of the posts elected by MPs.

Shadow Foreign Secretary Emily Thornberry strongly defended Corbyn, saying it was wrong to criticise him for being “too decisive” as she insisted the issue of elected posts was still on the table.

“It’s not a question of ‘forget all that’, there are negotiations going on. There’s an NEC [National Executive Committee] away-day in which this issue is going to be discussed as part of a larger package in terms of making sure that the party is more democratic, and these negotiations are ongoing. What do you want?

“The problem is that on the one hand people criticise, and have been criticising, Jeremy for being weak, for taking too long on his reshuffles, taking a couple of days, and yet when he decides that he will do a reshuffle that he needs to do in order to fill vacancies and in order to reach out, people then criticise him for being too decisive and too strong.”

Thornberry also dismissed criticism that the top four positions in the shadow cabinet are held by north London MPs: “Half of the shadow Cabinet come from the Midlands and the North, what is your problem?”